Toyota's Prius hybrid is becoming a little less quiet with a new electronic humming device that is the automaker's answer to complaints that pedestrians can't hear the top-selling car approaching.

The 12,600 yen (£96) speaker system that goes under the hood of the third-generation Prius sets off a whirring sound designed to be about the same noise level as a regular car engine so that it isn't annoying, Toyota Motor Corp. said today.

It goes on sale 30 August in Japan, and owners pay extra for installation charges. Its use is voluntary.

Overseas sales plans are still undecided, but Toyota is studying regulations and considering offering it in the US and other markets, said spokeswoman Monika Saito.

The gasoline-electric hybrid gets good mileage but is also quiet because it runs as an electric car much of the time. That advantage has drawn complaints that pedestrians, the blind in particular, are at greater risk of being hit by the car, especially at low speeds.

The US government's auto safety agency found in a research report last year that hybrids are twice as likely to be involved in pedestrian crashes at low speeds compared with cars with conventional engines.

Toyota, which also makes the Camry sedan and Lexus luxury models, said it plans versions of the device for other hybrid models, plug-ins, electric vehicles and fuel-cell vehicles.

Pedestrian deaths compared to overall traffic fatalities are higher in Japan than in the US and many other nations because of Japan's narrow and crisscrossing crowded streets. Japan is also a rapidly aging society, making audible cars critical.

Toyota said the device is based on guidelines addressing the dangers of silent cars, including hybrids, issued in January by the Japanese government.

Other automakers, including General Motors Co., Ford Motor Co. and Nissan Motor Co., are also working on countermeasures to make quiet ecological cars safer.

The Prius device's humming is so soft it is barely audible in a noisy street but can be a lifesaver in quieter environments. It can be turned off with a switch but goes on automatically every time the car starts.

The Prius has been the top-selling car in Japan for the past 15 months straight, benefiting from incentives designed to boost sales of green cars.

Toyota, the world's biggest automaker, has sold nearly 337,000 third-generation Prius cars in Japan. It has sold more than 2.68 million hybrids around the world so far, a million of them in Japan.